This invention relates to telecommunications systems and, more particularly, to a method and apparatus for adaptively quantizing a signal prior to digital-to-analog conversion during pulse shaping in a pulse amplitude modulated system.
Pulse amplitude-modulated (PAM) systems involve the transmission of pulse-shaped impulses over a transmission channel. The amplitude of a particular impulse relates directly to the information carried in the impulse. In general, a PAM signal may be expressed by the following expression:                               y          ⁡                      (            t            )                          =                                            ∑                              n                =                                  -                  ∞                                                      ∞                    ⁢                                    c              n                        ⁢                          p              ⁡                              (                                  t                  -                  nT                                )                                                                        Equation  1.0            
where cn represents the amplitude of the nth pulse and p(t) is the pulse shaping waveform, In many modern digital communication systems pulse-shaping is performed digitally. There are several reasons for using digital pulse shaping. Because of matched filtering requirements at the receiver and spectral shaping requirements for electromagnetic compatibility, it is often easier to implement the desired pulse waveform digitally than in the analog domain. Additionally, because attainable sampling frequently affects the ease of design of both the analog-to-digital converter and image rejection filter, digital pulse shaping may be used to increase sampling rate and allow greater ease of design.
Modern digital communication systems may typically use channel aggregation or multicarrier modulation on a transmission link. Certain of these modern digital communication systems include the use of individual channel gain adjustment on the separate channels in order to provide enhanced performance for coherent reverse link reception and quality of service features. However, individual channel gain adjustment results in power adjustment of incoming data, which when combined with pulse shaping, can dramatically affect resultant signal power, and a transmitted signal on a multicarrier link may have a higher peak-to-average ratio in than in a single carrier system. A higher peak-to-average ratio generally requires higher precision and a digital-to-analog converter (DAC) using more bits. In a DAC the signal level must be adjusted to make use of the full conversion range. A higher signal level results in clipping noise. However, using a lower signal level to avoid clipping results in a waste of DAC precision with the quantization power causing a significant amount of quantization noise. As a result, the effective spurious free dynamic range (SFDR) of a signal may degrade in a multicarrier system if quantization effects or clipping distortion due to digital-to-analog conversion are not taken into account. A degradation in SFDR can negatively affect the adjacent channel power rejection (ACPR) in digital pulse-amplitude modulated communication system causing greater intra-channel interference.
The present invention provides a method and apparatus for adaptively quantizing a digital signal prior to digital-to-analog conversion during pulse shaping in a pulse-amplitude modulated (PAM) system. Implementation of the invention provides a performance advantage in terms of adjacent channel power rejection (ACPR). The method and apparatus utilizes the inherent cyclostationarity of an input pulse-amplitude modulated (PAM) transmission in an adaptive quantization method. In the adaptive quantization method, quantization constants applied to the input PAM signal prior to digital-to-analog conversion change according to the time-varying periodic probability density function (pdf) of the input for PAM signal. The adaptive quantization is optimized for the instantaneous pdf of the input PAM signal, rather than overall possible distributions of the pdf. Spectral components added to the PAM signal due to the adaptive scaling of the method and apparatus may be removed by performing adaptive amplification on the analog output signal of the DAC before transmission. The adaptive amplification prevents the need to transmit scaling data to the receiver.
In an embodiment of the invention, the method and apparatus is implemented in a transmitter of a PAM system transmitting at least one channel over an air interface to a receiver. In the embodiment, signals for at least one channel are digitally processed to generate an information signal and passed through a digital pulse-shaping filter. The output of the digital pulse-shaping filter is then adaptively quantized by processing the filter output through an adaptive scaler and digital-to-analog converter (DAC). The adaptive quantization is performed by determining scaling factors that are used to scale the output of the digital pulse-shaping filter before the signal is input to the DAC. The scaling factors are derived from the probability density function (pdf) of the signal at the output of the digital pulse-shaping filter. The scaling factors are derived by minimizing the mean-squared error between the DAC output signal and DAC input signal over each pdf that exists for the input signal. In order to remove any spectral distortion caused by scaling, the scaled signal may be rescaled after digital-to-analog conversion in the DAC; the resealing may be performed by multiplying the converted signal by 1/k(l) by a lowpass analog filter before transmission.